A superabsorbent polymer (SAP) is a synthetic polymer material having a function of absorbing water at about 5 hundred times to about 1 thousand times its own weight, and it has been differently called a superabsorbency material (SAM), an absorbent gel material (AGM), and so on by development enterprises.
The SAP disclosed above was started to be commercialized for sanitary items, and is now being widely used as a water combination soil for horticulture, a water-stop material for civil engineering and construction, a nursery sheet, a freshness preservative in a food distribution field, a poultice material, and the like in addition to the sanitary fittings such as a disposable diaper and a sanitary napkin.
In most cases, the SAP is being widely used in the field of sanitary fittings such as disposable diapers and sanitary napkins, and, for this purpose, it needs to show high absorptivity to moisture, the moisture absorbed therein must not leak out even under external pressure, and it needs to show excellent permeability by maintaining its shape even in an expanded (swelled) state after absorbing water.
However, it has been known that the water retention capacity (centrifuge retention capacity, CRC) indicating basic absorptivity and water retention ability of the SAP and the absorption ability under pressure (AUP) indicating the characteristic of holding the absorbed moisture even under external pressure are difficult to improve together.
This is because the water retention capacity may relatively increase but the absorption ability under pressure may decrease when the overall cross-linking density of the SAP is controlled to be low, because the cross-linked structure becomes loose and the gel strength is reduced.
On the other hand, when the cross-linking density is controlled to be high for improving the absorption ability under pressure, the basic water retention capacity may decrease because it becomes a dense cross-linked structure in which it is difficult to absorb moisture.
For this reason, there was a limit in providing the SAP of which the water retention capacity and the absorption ability under pressure are improved together.
To resolve this, there have been various attempts for improving such properties together by controlling kinds of inner cross-linking agents or surface cross-linking agents or usage of the same, but such attempts have reached the limit.
Therefore, developments of an SAP showing improved water retention capacity and absorption ability under pressure together and technology enabling the preparation of the same are continuously required.